The present invention relates to a current controlling device for an electromagnetic winding. In particular, it relates to a current controlling device for controlling a starting current and a holding current supplied to an electromagnetic winding of a solenoid valve and the like.
To Drive an electromagnetic appliance such as a solenoid valve, an electromagnetic nozzle or an electromagnetic relay, in general, a large starting current initially is supplied to an electromagnetic winding thereof, and then, the current is reduced to a controllable holding current lower than the starting current in order to hold the condition after start up.
FIG. 1 shows a prior controlling device to perform the above current control, and FIG. 2 shows a characteristic curve of current flowing through an electromagnetic winding of the circuit in FIG. 1. This current controlling device has been shown in the U.S. Pat. No. 4,345,296.
In those figures, when a driving signal 1 is not supplied, an input terminal 2 has zero potential. Consequently, transistors 3 and 5 are in off state, and a transistor 4 is in on state. Therefore, the current does not flow through an electromagnetic winding 6. A comparator 7 at its (+)input terminal is grounded through resistors 8 and 9, and a (-)input terminal thereof is supplied with a source voltage by means of an on state of a transistor 10. Therefore, the output level of the comparator 7 is in L level (low level), and a transistor 11 is cut off. If the driving signal 1 appears at the input terminal 2 at the time point t.sub.0 of FIG. 2, the potential of the input terminal 2 becomes high, thus the transistors 3, 5 are conductive, and the transistors 4, 10 are cut off. When the transistor 10 is cutt off, the (-)input terminal of the comparator 7 is supplied with the source voltage divided by resistors 12 and 13. When the transistor 5 is conductive, a starting current IA flows through the electromagnetic winding 6 and rises gradually, thereby causing the voltage drop across the resistor 9 to increase. If the starting current IA rises to the maximum value IA.sub.max, the voltage applied to the (+)input terminal of the comparator 7 exceeds the voltage applied to the (-)input terminal thereof, and the output level of the comparator 7 becomes H level (high level), thereby causing the transistor 11 to be conductive. Therefore, a zener diode 14 is inserted in parallel into the series connection of the base-emitter circuit of the transistor 5 and the resistor 9. Consequently, when the zener voltage is applied to said series connection, the current which flows through the electromagnetic winding 6 decreases from IA.sub.max to a lower holding current IH. If the driving signal 1 comes to an end, the potential at the input terminal 2 becomes zero, and the circuit is returned to the initial condition.
However, the prior controlling circuit has disadvantages as described below.
According to the prior art, since the starting current IA rises freely to the maximum value IA.sub.max thereof, the heat loss in the electromagnetic winding increases, and undesirable heating may be produced. That is to say, in order to reduce the heat loss, it is preferable that the maximum value IA.sub.max of the starting current be a lower value. In the prior art, however, the reduction of the maximum value IA.sub.max is accompanied by narrowing of the starting period T, and results in obstruction of the driving of, for example, a solenoid valve to be controlled. Consequently, in the prior art, since the reduction of the maximum value IA.sub.max is difficult, heat loss is increased because of the large starting current.
Also, in the prior art, the large current flows until the starting current attains the maximum value IA.sub.max even if the solenoid value has been completely opened before the end of the starting period T, causing heat loss to increase still more.
Further, in the prior art, since the reference voltage of the comparator 7 is obtained by dividing the source voltage by the resistors 12 and 13, the reference voltage is easily varied with the fluctuation of the source voltage. Therefore, the characteristics of the current flowing through the electromagnetic winding vary, and the stable control of the solenoid valve may be obstructed.